Brushless direct current (BLDC) motors may be used for appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation applications. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. A more detailed synopsis of BLDC motors may be found in Microchip Application Note AN857, entitled “Brushless DC Motor Control Made Easy;” and Microchip Application Note AN885, entitled “Brushless DC (BLDC) Motor Fundamentals;” both at www.microchip.com; and wherein both are hereby incorporated by reference herein for all purposes.
Referring to FIG. 1, depicted is a waveform diagram of motor phase currents and back electromotive force (BEMF) voltages in a sinusoidal drive. Drive commutation for a BLDC motor may be determined by monitoring the back electromotive force (BEMF) voltages at each phase (A-B-C) of the motor. This is referred to as “zero-crossing” where the BEMF varies above and below the zero-crossing voltage over each electrical cycle. The BEMF frequency is equivalent to the motor speed.
Generally, three-phase BLDC motors use mostly trapezoidal drive, e.g., 120 or 180 degrees drive. A trapezoidal drive induces more torque vibrations and therefore more acoustic noise than a pure sinusoidal drive. The efficiency of the BLDC motor using trapezoidal drive is also lower compared to using sinusoidal drive. Referring to FIG. 2, depicted is a schematic block diagram of a typical BLDC motor drive using pulse width modulation (PWM) to control three high side power MOSFETs and three low side power MOSFETs. Position sensors, e.g., external Hall effect sensors, are used for rotor position and speed determination. However, the trend in controlling BLDC motors is to go sensor-less to keep costs down. There are two main BEMF sensor-less sensing methods, more fully described in Microchip Application Note AN1160, entitled “Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function,” at www.microchip.com; and is hereby incorporated by reference herein for all purposes.
Referring to FIG. 3, depicted are schematic connection and timing diagrams for trapezoidal drive of a BLDC motor using sensor-less measure of the BEMF on an un-driven phase (floating phase). A problem with using sensor-less sinusoidal drive is that the BEMF voltage needs to be sensed while the motor is rotating which requires opening a phase to measure the phase voltage referenced to a common point (voltage). Unfortunately, opening a phase creates harmonics in the sine wave drive that results in mechanical vibrations and efficiency loss in the BLDC motor. Most sensor-less sinusoidal drive uses vector control that requires a lot of dedicated hardware as well as high computational power. The vector control uses many phase voltage samples per electrical turn.
There is one BLDC control method that uses a sensor-less sinusoidal drive technique where the three phases are always driven and there is no floating phase. Advantages of sinusoidal drive control are lower noise, lower mechanical vibration and higher efficiency. Drawbacks, however, are higher drive logic complexity; drive speed is limited depending upon the BLDC motor type, driving eight-pole motors are not possible, and weaknesses in motor stall detection. Referring to FIG. 4, depicted is a waveform diagram of current zero-crossing information that is used to get an approximated BEMF (horizontal line shown in FIG. 4). At least two zero-crossings are required (circles on horizontal line shown in FIG. 4), which create limitations and weaknesses in the use of this method of BLDC motor BEMF sensing and control.